Reluctance machines are well known in the art. In general a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move to a position where the inductance of an excited winding is maximized (i.e. the reluctance is minimized).
In one type of reluctance machine the phase windings are energized at a controlled frequency. This type of reluctance machine is generally referred to as a synchronous reluctance machine. In another type of reluctance machine, circuitry is provided to determine the position of the machine's rotor, and the windings of a phase are energized as a function of rotor position. This type of reluctance machine is generally referred to as a switched reluctance machine. Although the description of the current invention is in the context of a switched reluctance machine, the present invention is applicable to all forms of reluctance machines, including synchronous and switched reluctance motors and to other machines that have phase winding arrangements similar to those of switched reluctance machines.
The general theory of design and operation of switched reluctance machines is well known and discussed, for example in The Characteristics, Design and Applications of Switched Reluctance Motors and Drives, by Stephenson and Blake and presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
In general, reluctance machines have been designed with a stator yoke having inward projecting salient poles and a hollow core area. Nested concentricity in the hollow core area, or stator bore, is a rotor having outwardly projecting salient poles. Typically, the rotor contains no circuitry or permanent magnets. The rotor and the stator are coaxial. The rotor is connected to a rotor shaft which is free to rotate and acts as an output shaft when the machine is motoring, and as an input shaft when the machine is generating.
Associated with each stator pole is a coil of wire wound around the pole. The stator poles which are positioned opposite one another are generally coupled to form a single phase. A phase is energized by delivering current to the coil. Switching devices are generally provided which allow the coil to be alternately connected into a circuit which delivers current to the coil when the phase is energized and one which separates the coil from a current source when the phase is de-energized, and which may recover energy remaining in the winding.
Reluctance torque is developed in a reluctance machine by energizing a pair of stator poles when a pair of rotor poles is in a position of misalignment with the energized stator poles. The degree of misalignment between the stator poles and the rotor poles is called the phase angle. Energizing a pair of stator poles creates a magnetic north and south in the stator pole pair. Because the pair of rotor poles is misaligned with the energized stator poles by some phase angle, the inductance of the stator and rotor is less than maximum. The pair of rotor poles will tend to move to a position of maximum inductance with the energized windings. The position of maximum inductance occurs where the rotor and stator poles are aligned.
At a certain phase angle in the rotation of the rotor poles to the position of maximum inductance, but before the position of maximum inductance is achieved, the current is removed from the phase de-energizing the stator poles. Subsequently, or simultaneously, a second phase is energized, creating a new magnetic north and south pole in a second pair of stator poles. If the second phase is energized when the inductance between the second pair of stator poles and the rotor poles is increasing, positive torque is maintained and the rotation continues. Continuous rotation is developed by energizing and de-energizing the stator poles in this fashion. The total torque of an reluctance machine is the sum of the individual torques described above.
One problem associated with reluctance machines is that the torque developed by the machine is not smooth. Torque drops off steeply when the phase angle of the rotor is between the poles of the stator, when the inductance is minimized, then increases as the phase angle of the rotor moves toward alignment with a stator pole, when inductance is maximized. This rising and falling torque phenomenon is known as "torque ripple."
In the past the problem of torque ripple has sometimes been addressed by modifying the motor control circuitry. As one example, by profiling the current in a phase during the active time period when the phase is energized, the rate of change in the magnetic flux can be controlled resulting in less abrupt changes in machine torque. This approach requires complex circuitry, and therefore results in higher design, manufacturing, and maintenance costs.
In addition to the problem of torque ripple, known reluctance machines often produce undesirable noise and vibration. As the inductance of a reluctance machine increases and decreases, the magnetic flux in parts of the machine changes in relation to the increasing and decreasing inductance. As a typical reluctance machine's pair of rotor poles moves into a position of alignment with a pair of stator poles, lines of magnetic flux deform the shape of the rotor and stator poles, decreasing the separation space between the poles. The changing flux results in fluctuating magnetic forces being applied to the ferromagnetic parts of the machine. These forces can produce unwanted noise and vibration. One major mechanism by which these forces can create noise is the ovalizing of the stator caused by magnetic forces normal to the air-gap. Generally, as the magnetic flux increases along a given diameter of the stator, the stator is pulled into an oval shape by the magnetic forces. As the magnetic flux decreases, the stator pulls or springs back to its undistorted shape. This ovalizing and springing back of the stator will produce audible noise and can cause unwanted vibration.
Another problem associated with some reluctance machines is high core losses. When the phase windings of a reluctance machine are energized and de-energized, the magnetic field flux varies. The result of the variations is a loss of energy in the iron core of the machine. These core losses consist of eddy-current losses and hysteresis losses. As the rate the phases are energized and de-energized increases per unit of time (the commutation frequency) the hysteresis portion of the core losses increases proportionately, and the eddy-current losses increase proportionately to the square of the commutation frequency. Because many reluctance machines require relatively high commutation frequencies, the core losses can be significant as the magnet flux in a given magnet core varies at a rate proportional to the commutation frequency.
The above mentioned difficulties and limitations are common to most all known reluctance machine including those machines having a flux paths that are "radial" (i.e., generally perpendicular to the rotor's axis of rotation) and those machines having "axial" flux paths (i.e., flux paths that are generally parallel to the rotational axis of the rotor). In addition to suffering from the shortcomings described above, known axial flux reluctance machines are further limited in other respects.
FIGS. 1A and 1B illustrate a conventional axial flux machine 10 including a disc-shaped rotor 11 defining two rotor poles 11a and 11b and a stator 12 having two stators 12a and 12b defining three U-shaped stator poles 12a, 12b and 12c. FIG. 1A illustrates a side view of this conventional machine while FIG. 1B illustrates the cross-section of the rotor 11 and one of the stators. As illustrated, opposing stator teeth from the two stators are aligned such that all of the torque is produced as a function of the self-inductance of the phase windings and such that the only significant torque producing flux paths extend from one of the stators, through the active rotor material to and through the other stator.
Conventional disc-type axial flux reluctance machines having U-shaped poles such as the one illustrated in FIGS. 1A and 1B suffer from the previously described shortcomings, but suffer more acutely from high core losses as the flux flowing through the stator poles is the same as the flux flowing through the stator yoke. This is clearly reflected by FIG. 1B where the extensive length of each stator pole is shown and the flux path extends through the stator poles and the stator yoke. Thus, because all of the pole flux passes through the stator yoke, the stator yokes of such machines must be relatively large to maintain reasonable flux densities.
In addition to suffering high core losses, conventional axial flux reluctance machines, like machine 10, are limited in that they are often difficult to construct and are limited to low torque applications. The construction difficulties arise from the fact that the rotor must comprise both paramagnetic portions formed from, e.g., iron or steel 11a and 11b (to carry magnetic flux) and non-paramagnetic separating material 14 to separate the flux carrying portions of the rotor. This non-magnetic separating material 14 is required to set up the torque-producing flux paths necessary for proper operation of such conventional machines. The construction of such a two-material rotor is often complex and expensive. Moreover, because only a portion of the rotor contains active torque-producing material, the torque output of such a machine is typically far less than would be desired for a rotor of a given size. The above described difficulties are also associated with the stators of conventional axial flux reluctance machines which are often constructed from both paramagnetic and non-paramagnetic materials.
It is an object of the present invention to overcome these and other limitations of the prior art by, among other things, providing an axial flux reluctance machine and a method of operating a reluctance machine whose torque ripple, noise, vibration, and core loss characteristics are better than those available from known reluctance machines and operating methods.